In general, radio wave transceiver devices all integrate a stage allowing mixing of a received signal with a reference signal coming from a frequency synthesizer. This mixing stage is often made up of a mixer and a local oscillator integrated into a phase locked loop (PLL). Such a local oscillator uses a technology chosen from among several possible technologies (quartz, acoustic waves, LC circuit, oscillator ring, etc.) based on the desired performance/cost characteristics for the device.
However, whatever the chosen technology, the known oscillators lead to radiofrequency devices with two major problems: on the one hand, the volume of the device, which remains too large; on the other hand, its frequency tuning range is relatively limited (smaller than an octave). In particular, faced with the multiplication of telecommunications standards, a limited tuning range represents a particularly significant obstacle, preventing the possible use of “multi-standard” and/or opportunistic devices.
Consequently, highly integrated radiofrequency devices with a wide frequency band, which are essential to pursue miniaturization of the radiofrequency circuits integrating them, can only be developed based on new types of oscillator.
However, a new type of oscillator, called magnetic, was recently proposed. The operation of a magnetic oscillator is based on the spin-transfer torque physical phenomenon.
The spin-transfer torque phenomenon manifests itself in components with a structure successively including a first ferromagnetic layer (from several nm to several tens of rim thick), a non-magnetic intermediate layer, and a second ferromagnetic layer.
In particular, this spin-transfer torque phenomenon between the charge carriers (electrons or holes) exists in structures called nanopillars made up of a first fine ferromagnetic layer (typically from several nm to several tens of nm), a non-magnetic intermediate layer, and a second fine ferromagnetic layer, these layers having a reduced size in a plane orthogonal to the stacking direction of the layers. Typically, a nanopillar is a cylinder whereof the axis corresponds to the stacking direction of the layers and whereof the radius is typically several tens of nanometers.
The injection of a current through this structure makes it possible to generate a spin-polarized current. The interaction between the spins of the charge carriers (electrons or holes) of the spin-polarized current and the magnetization of the ferromagnetic material of one of the magnetic layers, for example the second layer, results in a torque, called “spin transfer torque”, related to the relaxation of all or part of that spin polarization on the magnetization.
The spin transfer phenomenon thus makes it possible to manipulate the magnetization of the second layer without applying a magnetic field, but by applying a power supply current that has been spin polarized. This principle is implemented in the latest generations of MRAM (“Magnetic Random Access Memory”) to switch the magnetic configuration in which the magnetic information is stored.
The spin-transfer torque phenomenon therefore causes a modification in the magnetic configuration. This may be detected through a variation of the electrical resistance of the nanopillar. In fact, the electrical resistance of the nanopillar depends on the orientation of the magnetization of the second layer relative to that of the first layer. This magnetoresistive effect is referred to as giant magnetoresistance (GMR) in all-metal nanopillars, and tunnel magnetoresistance (TMR) in nanopillars with tunnel junctions.
The magnetic configuration of a ferromagnetic layer in a nanopillar having nanometric dimensions is a magnetic mono-domain. In fact, the insertion of a wall between two domains having different magnetizations is too expensive in terms of energy, such that such a magnetic configuration is not seen.
However, the residual magnetic configuration, which is the stable configuration resulting from a competition between the exchange energy and the magnetostatic energies, is not necessarily a configuration in which the magnetization is uniform. Another possible type of remnant magnetic configuration corresponds to a so-called “vortex” magnetization. In such a configuration, the magnetic moments, which are essentially planar, wind around the center of the ferromagnetic layer, except in a region surrounding the center of the layer, called vortex core and having a radius of the order of magnitude of the exchange length (LEX) of the material (for example, 5-6 nm in Co or the NiFe alloy), where the magnetic moments of the magnetic configuration point outside the plane of the layer, upward or downward. The direction in which the magnetization of the core points defines the polarity of the vortex (P=+/−1). A second parameter of the vortex is its chirality, which corresponds to the winding direction of the magnetic moments around the center of the layer and which may assume two values; C=+1 when they wind in the clockwise direction, or C=−1 when they wind in the opposite direction. A ferromagnetic layer may be in one of the four possible vortex configurations corresponding to a value of the polarity and a value of the chirality. These four configurations are shown in FIG. 1.
Armed with this observation, patent application US 2009/0117370 A1 describes, in the field of magnetic non-volatile memories, the use of a nanopillar including a pattern comprising two magnetic layers, each of them having a residual magnetic vortex configuration. The four chirality configurations of the vortices make it possible to produce a memory with four states. This US patent does not use the polarity of the vortices as a storage degree for the information. Owing to the effects of giant or tunnel magnetic resistance, the measurement of the total resistance of the nanopillar makes it possible to determine the current state of the memory.
Aside from the effects making it possible to modify the static magnetic configurations presented above, the spin-transfer torque phenomenon may also cause (under certain conditions related to the outside magnetic field in which the nanopillar is placed and the intensity of the power supply current injected through it) a continuous oscillation of the magnetic configuration of one or both magnetic layers. This oscillation corresponds to the excitation of dynamic modes related to the static magnetic configuration (uniform, C-state, vortex, etc.), whereof the characteristic frequencies depend on the parameters related to the choices of the component materials of these magnetic layers (saturation magnetization, exchange length, damping coefficient, etc.), as well as the dimensions (radius and thickness for disk-shaped layers) and geometry of the pillar.
Owing to the magnetoresistive effect previously described, any oscillation of all or part of the magnetization of the structure may cause a variation of the resistance of the pillar at a frequency equal to that of the oscillation of the magnetization. An oscillating voltage is then obtained across the terminals of the pillar, the frequency spectrum of which reflects that of the excited magnetic modes of the structure.
This oscillation effect will henceforth be expressed through the microwave frequency power emitted by the nanopillar, which corresponds to the product of the voltage across the terminals of the nanopillar multiplied by the intensity of the power supply current injected through it.
The characteristic frequencies are therefore specific to the nanopillar and may be modulated by acting on the static magnetic configuration. They thus depend on the amplitude of the outside field, the intensity of the power supply current (typically approximately a milliampere, which corresponds to current densities of approximately 107 A/cm2 through a section of the nanopillar), dimensions of the layers of the nanopillar. The range of accessible characteristic frequencies is comprised between several hundred MHz and several tens of GHz.
A component made up of such a nanopillar and suitable means for injecting a power supply current and applying an outside magnetic field so as, by using the spin-transfer torque phenomenon, to place them in a pillar in an excited state corresponding to a continuous oscillation of the magnetic configuration of one or the set of both magnetic layers, constitutes a magnetic oscillator. Thus, for example, in the article by Locatelli et al., it is provided to apply a power supply current for vortex nucleation in the first and second layers of the oscillator, the residual magnetic configuration of which is uniform.
Compared to other existing technologies, such a magnetic oscillator has several advantages such as smaller dimensions, a larger accessible tuning range owing to the variation of the amplitude of the power supply current and an adapted outside magnetic field, change speed of the frequency operating point (for example through adaptive variations of the intensity of the power supply current), and lack of sensitivity to electromagnetic radiation.